Method for performing packet switched handover in a mobile communication system

ABSTRACT

The invention relates to a method and system for performing packet switched handover in a mobile communication network. The system comprises a mobile node, a first and a second packet switching node. The method enables the parallel sending of logical link layer frames from the first and the second packet switching node. This is achieved so that the mobile node does not reject incoming frames received from two logical link layer entities having different states. The benefits of the invention are related to improved quality of service and the avoiding of gaps in received data during handover.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to mobile communication systems. Particularly, theinvention relates to the performing of packet switched handover in amobile communication system.

2. Description of the Related Art

The introduction of conversational and streaming services in GlobalSystem of Mobile Communications (GSM) has created a demand for efficienthandovers from user perspective in GSM/Edge Radio Access Network(GERAN). The General Packet Radio Service (GPRS) and the IP MultimediaSystem (IMS) support the conversational and streaming services on theirside and impose requirements on the GERAN side. It is necessary to beable to perform Packet Switched (PS) handovers frequently enough and tobe able to minimize interruptions in a constant packet stream to amobile terminal. The interruptions must preferably be short enough toenable a packet buffering mechanism in the mobile terminal to hide theinterruptions. Previously in GPRS it was sufficient to provide aloss-free link layer service for interactive applications such asWireless Application Protocol (WAP) browsing. In browsing applicationsmoderate extra delays caused by handovers are acceptable. However, instreaming or conversational class services interruptions in thesupposedly constant packet stream are immediately noticeable unless, ofcourse, they can be hidden using large enough buffers in the receivingends. However, such buffering introduces always a delay in the mediastreams provided to the user. In the case of conversational voiceservices any significant delays are unallowable, especially consideringother factors already introducing a delay in the speech path such asnoise filtering and speech coding.

Reference is now made to FIG. 1, which is a block diagram illustratingthe architecture and the protocol stacks in a GPRS system in associationwith the GERAN. The GPRS system is specified, for example, in the 3GPPspecification 23.060. The protocol stacks are illustrated from the userplane point of view. In FIG. 1 there is a Gateway GPRS Support Node(GGSN) 106. GGSN 106 is connected to an external network (not shown) viaa Gi interface. The external network may be an arbitrary IP network, forexample, the Internet or an intranet. In FIG. 1 there is also a ServingGPRS Support Node (SGSN) 104. GGSN 106 communicates with SGSN 104, whichroutes packets to and from Mobile Station (MS) 100 via a Base StationSubsystem (BSS). SGSN 104 takes care of the mobility related tasks suchas the maintaining of mobile station 100 location information, networkregistrations, routing area and location updating, Packet Data Context(PDP) activation and deactivation, handovers and the paging of mobilestation 100. Part of the above mentioned tasks are naturally done inother network elements with which SGSN 104 is communicating. The GGSN isresponsible for routing and tunneling packets to and from a number ofSGSN 104 and other SGSNs. The routing is based on SGSN addressinformation maintained in a PDP context information held by GGSN 106 foreach network address activated for MS 100, for example, an IP address oran X.25 address or a PPP link.

In FIG. 1, the uppermost protocol layer in MS 100 is the applicationlayer (APPL). The application layer may be any protocol, for example, aWAP protocol or Transmission Control Protocol (TCP) or UniversalDatagram Protocol (UDP). Over TCP/IP may be carried, for example,Hypertext Transfer Protocol (HTTP). The application layer communicationis exchanged with a peer host, which may be located behind the Giinterface, for example, in the Internet. Below the application layerthere is the IP layer or alternatively X.25 layer, which in GPRS issupported by both MS 100 and GGSN 106. The IP address for packetsaddressed to MS 100 points to GGSN 106. An IP packet 114 is conveyed toMS 100 using GPRS user plane protocols below the IP layer. Between GGSN106 and SGSN 104 IP packet 114 is conveyed using the GPRS TunnelingProtocol (GTP). A GTP packet carried further over UDP/IP.

In SGSN IP packet 114 data is routed based on MS 100 locationinformation and passed to Sub-Network Dependent Convergence Protocol(SNDCP) layer. SNDCP is specified in the 3GPP specification 44.065.SNDCP layer maps network-level characteristics onto the characteristicsof the underlying network. For example, SNDCP takes care of thetransmission and reception of Network layer Protocol Data Units (N-PDU)carrying IP packets. For example, IP packet 114 is carried in N-PDU 112.SNDCP multiplexes several packet data protocol packets for the same MS.It segments IP packet 114 to LLC frames, for example, LLC frame 110. Italso reassembles packets from LLC frames. Header compression and packetpayload compression is also performed at SNDCP layer. SNDCP performsparameter negotiation between MS 100 and SGSN 104. SNDCP buffers N-PDUsin the case of acknowledged mode services.

The Logical Link Control (LLC) layer provides a highly reliable linkbetween MS 100 and SGSN 104. The LLC is specified in 3GPP specifications44.064 and 04.64. The LLC is independent of the underlying radioprotocols and hides the BSS and radio interface related tasks from theLLC layer users. LLC supports variable-length information frames. LLCsupports both acknowledged and unacknowledged data transfers, that is,acknowledged and unacknowledged modes of operation. LLC providesservices typical to a link layer comprising parameter negotiation, flowcontrol in the Asynchronous Balanced Mode (ABM), sequence control tomaintain the ordering of LLC-frames, expedited delivery forhigh-priority data, error detection, error recovery and indication. LLCperforms data confidentiality by means of the ciphering of LLC-framecontents. LLC also supports user identity confidentiality by means ofthe use of Temporary Logical Link Identity (TLLI) instead ofInternational Mobile Subscriber Identity (IMSI).

The relay layer relays LLC PDUs between the Um and Gb interfaces in theBSS. The Base Station System GPRS Protocol (BSSGP) layer specified in3GPP specification 08.18 conveys routing and QoS-related informationbetween the BSS and the SGSN. For example, it carries radio resourcerelated requests from the SGSN to the BSS 102. It also carries LLCframes between the BSS and the SGSN. In addition to LLC frames it alsocarries signaling PDUs associated with GRPS mobility management. TheNetwork Service (NS) layer transports BSSGP PDUs between BSS and SGSN.NS may be based on Frame Relay (FR). The RLC sub-layer within theRLC/MAC layer provides a radio technology dependent reliable linkbetween MS 100 and BSS 102. The MAC sub-layer performs the requestingand reservation of radio resources and maps LLC frames onto the GSMphysical channels. The task of the MAC layer is to ensure efficientsharing of common radio resources by several mobile stations. TheRLC/MAC layer is defined in the 3GPP specification GSM 04.60.

The standardization organization 3G Partnership Project (3GPP) iscurrently specifying the packet switched handover for GERAN A/Gb mode.One of the key aspects of the packet switched handover is duplicatedpacket forwarding to both a source BSS and a target BSS during handover,which has not yet been thoroughly covered in the specifications.

Reference is now made to FIG. 2, which is a block diagram of GPRSarchitecture illustrating problems in prior art associated withduplicated packet forwarding. According to current GPRS specifications,an LLC entity in a new SGSN can only be started so that an LLCconnection is establishing at the request of an SNDCP entity or the peerLLC entity. An LLC entity can only be created in its initial state wherethe LLC connection variables have their initial values. In FIG. 2 thereis an MS 100, Base Transceiver Stations (BTS) 224-228 and BaseController Stations (BSC) 210-214 in BSS 216. There is a GGSN 200, whichis connected to IP network 201. From IP network 201 is received adownlink packet stream 246 for which a real-time service is required.Initially, downlink packet stream 246 is tunneled to SGSN 202 as packetstream 240. Initially, SGSN 202 routes packet stream 240 to MS 100 viaBSC 212 and BTS 222 as packet stream 242 using an LLC connectionterminating at an LLC entity 230, which is located in MS 100. BSC 212and BTS 222 are referred to as source BSS 262. MS 100 communicates withBSC 212 via BTS 222. BSC 212 performs handover related tasks includingthe handover determination algorithms and decisions. In handover relatedsignaling an SGSN communicates with a BSC within a BSS. Similarly, inhandover related signaling an MS communicates with a BSC within a BSS.The signaling between an MS and a BSC goes via a BTS.

However, when MS 100 receives a report indicating that a cell served byBTS 224 has better radio quality, it must start performing handover tothe cell served by BTS 224. The new cell is under the area of a new SGSN204. After the handover, packet stream 246 should be routed to MS 100from GGSN 200 via SGSN 204, BSC 214 and BTS 224. BSC 214 and BTS 224 arealso referred to as a target BSS 264. While the handover is not fullycomplete, SGSN 202 must forward packets to both BSC 212 and SGSN 204. Inorder to be able to process packets from packet stream 240 SGSN 204 mustreceive them as a GTP tunneled packet stream 241 from SGSN 202. Packetsfrom GTP tunneled packet stream 241 are forwarded in SGSN 204 to its LLCentity 254. The LLC entity is started from initial state with initialLLC connection variables. GTP tunneled packet stream 241 is routed fromSGSN 204 as packet stream 244 carried over an LLC connection. Theproblem in the packet duplicated forwarding mechanism described above isthat LLC entity 254 in the new SGSN, namely SGSN 204, has differentstate compared to LLC entity 252 and LLC entity 230. This means that LLCentity 230 in MS 100 receives packets from two different independent LLCentities. The corresponding peer LLC entity 230 in MS 100 is not capableof receiving simultaneously packets from two different LLC entities, ifthe states of the LLC entities comprising the LLC variables are notsynchronized. The different states essentially lead to the rejection ofLLC frames carrying packet stream 244 or the receiving of duplicate LLCframes in an uncontrolled manner.

The rejection is due to the fact that LLC entity 252 sends LLC frameswith sequence numbers that are overlapping with the sequence numberssent by LLC entity 254 even though they are different LLC frames. Framesare rejected in LLC entity 230 also due to the fact that LLC entity 254sends LLC frames using different ciphering parameters. Because theciphering parameters are different, LLC entity 230 is unable to decipherthe LLC frames and discards them due to failing Frame Check Sequence(FCS) verification. A further problem is that SGSN 204 is unaware of theLLC frame sizes negotiated between MS 100 and SGSN 202. If SGSN 204 usesvalues that exceed the maximum values supported by MS 100, it discardsall LLC frames. This in turn may lead to the releasing of the PDPcontext carrying packets streams 240, 241, 242 and 244. MS 100 mayadditionally also perform reset.

As explained in the 3GPP specification 44.064, the ciphering parametersfor LLC frames comprise IOV, LFN, OC and SX. IOV is an Input OffsetValue, which is a 32 bit random value generated by the SGSN. LFN is theLLC Frame Number (LFN) in the LLC frame header. OC is an overflowcounter that is calculated and maintained independently at the sendingand the receiving sides. An OC for acknowledged operation must be set to0 whenever asynchronous balanced mode operation is re-established forthe corresponding Data Link Connection Identifier (DLCI). An LLC layerconnection is identified using DLCI, which consists of Service AccessPoint Identifier (SAPI) and the TLLI associated with MS 100. OC shall beincremented by 512 every time when the corresponding LFN rolls over. Dueto this fact, OC is never sent directly in LLC frames. The aim of OC isto add variation to the ciphering process in order to make it morerobust. SX is an XOR mask calculated from the LLC entity identifier.There are two IOV values, one for numbered information frames associatedwith acknowledged operation and another for unconfirmed informationframes associated with unacknowledged operation. There are also two LFNvalues, one for acknowledged operation and another for unacknowledgedoperation. There are four OC counters associated with each DLCI. Thereis one OC counter per operation mode, which is either unacknowledged oracknowledged, and direction of transmission, which is either uplink ordownlink.

Naturally, the session key K_(c) used in the ciphering algorithm is oneof the ciphering parameters.

Reference is now made to FIG. 3, which is a signaling diagramillustrating signaling during a packet switched handover in accordancewith the current 3GPP proposals. The current proposals are described inTSG document GP-032710 “Packet Switched Handover for GERAN A/Gb mode,Stage 2”, version 0.2.0, 2004-01. The architecture associated with thesignaling is as illustrated in FIG. 2. MS 100 sends radio qualitymeasurement information pertaining to neighboring cells to source BSS262 using message 301. Based on the measurement information source BSS262 determines that handover is required. At time t₀ source BSS 262determines that handover is to be performed to a new cell, which is inthe area of a new SGSN, which is SGSN 204. Source BSS 262 sends a PSHandover Required message 302 to old SGSN 202. The message comprises,for instance, the source cell, the target cell, TLLI, cause and atransparent container. SGSN 202 determines based on the target cell ifthe handover is an intra- or inter-SGSN handover. SGSN 202 determinesthe identity of the new SGSN and sends a Prepare PS Handover Requestmessage 303 to SGSN 204. SGSN 204 sends a PS Handover Required message304, which requests target BSS 264 to reserve radio resources for MS 100in the target cell. When radio resources have been successfullyallocated, target BSS 264 sends a PS Handover Request Acknowledgemessage 305 indicating successful allocation. SGSN 204 sends a PreparePS Handover Response message 306 to SGSN 202, which tells, among otherthings, that SGSN 202 may issue to MS 100 a command to complete handoverto the new cell. SGSN 202 receives message 306 at time t₁.

However, simultaneously a packet from GTP packet stream 307 is receivedby SGSN 202. SGSN 202 forwards packets one by one from GTP packet stream307 to SGSN 204 as packet stream 308. SGSN 204 sends packets from packetstream 308 further to target BSS 264 as packet stream 309. Target BSSforwards packets from packet stream 308 to MS 100 as packet stream 310.There is a delay before MS 100 is able to receive packets from SGSN 204via target BSS 264. SGSN 202 sends PS Handover Command message 311 tosource BSS 262. Source BSS sends further PS Handover Command message toMS 100. Thereupon, MS 100 tunes to the radio channel and timeslotallocated in the target cell by target BSS 264. This is illustratedusing arrow 312. Target BSS 264 sends Physical information to MS 100 forMS 100 to synchronize. After MS 100 has synchronized, it sends a PSHandover Complete message 314 to target BSS 264 at time t₂. Only aftertime t₂ MS 100 is prepared to receive packets via target BSS 264normally, which shows that there is an intolerable delay unless MS 100receives packets via both target BSS 264 and source BSS 262. Target BSS264 sends a PS Handover Complete message 315 to SGSN 204. Thereupon,SGSN 204 performs PDP context update messaging represented using arrows316 and 317 with GGSN 200. PDP context update indicates to GGSN 200 theaddress of current SGSN 204. After having received PDP context update attime t₃, GGSN 200 is able to start routing GTP packet stream 318 toright SGSN, which is now SGSN 204. Thereupon, MS 100 receives packetstream 320 from target BSS 264, which has received it from SGSN 204 aspacket stream 319.

Reference is now made to FIG. 4, which is signaling diagram illustratingthe delay associated with a solution, which merely forwards packets froma source node to a target node during handover processing. The solutionis similar to the solution utilized in UMTS in association with ServingRadio Network Server SRNS relocation. SRNS relocation is explained in3GPP 23.060. In FIG. 4 a source node 452 receives a packet stream 401sent by an upper node 450, which is connected to an IP network 451. Attime t₀ upper node sends a specific packet 460 in packet stream 401.Source node forwards packet stream further 402 to MS 100 via an accessnetwork 456. At time t₁ MS 100 decides to start using a target node 454instead of source node 452 for receiving packet streams. At time t₁ MS100 acknowledges last frame received via source node 452 using message403. Packet 460 has not been completely received, for example the lastframe from packet 460 may be pending. MS 100 sends a request message 403for source node 452 indicating the abandoning of source node 452 for MS100 traffic. After receiving message 403, source node 452 startsforwarding all packets addressed to MS 100 via target node 454 as packetstream 405. Packet stream 405 is forwarded by target node 454 to MS 100as packet stream 406. At time t₂ MS 100 receives a first packet since MS100 received the last frame via source node 452 at time t₁. The timedifference between t₁ and t₂ represents the gap in the receiving ofpackets at MS 100, whereas the time difference between t₀ and t₂represent a delay in receiving packet 460 from upper node 450 to MS 100.The delays explained above are intolerable for realtime services.

As has been illustrated in association with FIGS. 2, 3 and 4, there areproblems in performing packet switched handover using current GPRSarchitecture and the solutions proposed in prior art. On the one hand,it must be possible for an MS to receive packets simultaneously from asource node and a target node during the handover signaling. On theother hand, this is not possible in current GPRS specifications andleads to the rejection of forwarded frames at the MS side.

SUMMARY OF THE INVENTION

The invention relates to a method of performing handover in a mobilecommunication system comprising a mobile node, a first and a secondpacket switching node. In the method a handover condition associatedwith the mobile node is detected in the first packet switching node; thefirst packet switching node requests handover preparation from thesecond packet switching node; logical link layer information is receivedfrom the first packet switching node to the second packet switchingnode; the state in a logical link layer entity is set in the secondpacket switching node based on the logical link layer state information;and logical link layer frames are sent from the first and second packetswitching nodes to the mobile node during handover.

The invention relates also to a method performing handover in a mobilecommunication system comprising a mobile node, a first and a secondpacket switching node. In the method a handover condition associatedwith the mobile node is detected in the first packet switching node; thefirst packet switching node requests handover preparation from thesecond packet switching node; a packet is received at the first packetswitching node; a logical link layer Protocol Data Unit (PDU) is formedfrom data in the packet; a first frame containing the logical link layerProtocol Data Unit (PDU) is sent to the mobile node from the firstpacket switching node; the logical link Protocol data Unit (PDU) is sentfrom the first packet switching node to the second packet switchingnode; and a second frame containing the logical link layer Protocol DataUnit (PDU) is sent to the mobile node from the second packet switchingnode.

The invention relates also to a method performing handover in a mobilecommunication system comprising a mobile node, a first and a secondpacket switching node. In the method a handover condition associatedwith the mobile node is detected in the first packet switching node; thefirst packet switching node requests handover preparation from thesecond packet switching node; at least one ciphering parameter isreceived from the first packet switching node to the second packetswitching node; a logical link parameter exchange is performed betweenthe mobile node and the first packet switching node; and logical linklayer frames are sent from the first and second packet switching nodesto the mobile node during handover.

The invention relates also to a method performing handover in a mobilecommunication system comprising a mobile node, a first and a secondpacket switching node. In the method a first logical link layer entityis formed in the mobile node; a handover condition is detected in themobile node; a second logical link layer entity is formed in the mobilenode; logical link layer frames are sent from the first and secondpacket switching nodes to the mobile node during handover; handovercompletion is detected; and logical link layer parameters between themobile node and the second packet switching node are renegotiated afterthe detecting of the handover completion if the logical link layerparameters are not suitable.

The invention relates also to a system, which comprises a mobile node, afirst and a second packet switching node. The system further comprises:signaling means in the first packet switching node for detecting ahandover condition associated with the mobile node, requesting handoverpreparation from the second packet switching node and sending logicallink layer information to the second packet switching node; signalingmeans in the second packet switching node for receiving logical linklayer information from the first packet switching node; control means inthe second packet switching node arranged to set the state in a logicallink layer entity based on logical link layer information from the firstpacket switching node; and control means in the first packet switchingnode arranged to send logical link layer frames to the mobile nodeduring handover.

The invention relates also to a system, which comprises a mobile node, afirst and a second packet switching node. The system further comprises:signaling means in the first packet switching node for detecting ahandover condition associated with the mobile node and requestinghandover preparation from the second packet switching node; logical linklayer means in said first packet switching node for forming logical linklayer Protocol Data Units (PDU) and sending said logical link layerProtocol Data Units (PDU) to said second packet switching node; andlogical link layer means in said second packet switching node forsending said logical link layer Protocol Data Units (PDU) transparentlyto said mobile node.

The invention relates also to a system, which comprises a mobile node, afirst and a second packet switching node. The system further comprises:signaling means in the first packet switching node for detecting ahandover condition associated with the mobile node, requesting handoverpreparation from the second packet switching node and sending at leastone ciphering parameter to the second packet switching node; signalingmeans in the second packet switching node for receiving at least oneciphering parameter from the first packet switching node; logical linklayer means in the first packet switching node for performing a logicallink parameter exchange with the mobile node.

The invention relates also to a system, which comprises a mobile node, afirst and a second packet switching node. The system further comprises:control means in the mobile node arranged to form a first logical linklayer entity in response to connection establishment and a secondlogical link layer entity in response to a handover condition; signalingmeans in the mobile node for detecting the handover condition and ahandover completion; logical link layer means in the mobile nodearranged to renegotiate logical link layer parameters with the secondpacket switched node after the handover completion if the logical linklayer parameters are not suitable.

In one embodiment of the invention, the mobile node is a mobileterminal, for example, a UMTS terminal, a GSM terminal, a GPRS terminal,a WLAN terminal or a terminal within an arbitrary cellular radio system.

In one embodiment of the invention, the mobile node is a mobilecomputer, for example, a laptop computer, palmtop computer or a personaldigital assistant (PDA).

In one embodiment of the invention, the mobile communication system is aGeneral Packet Radio Service (GPRS), the first and second packetswitching nodes are Serving GPRS Support Nodes (SGSN) and the logicallink layer is GPRS Logical Link Control (LLC) and the logical linkparameter exchange is Logical Link Control (LLC) exchange Identification(XID) negotiation. In one embodiment of the invention the second packetswitching node is a Base Station Subsystem (BSS) node, for example, abase station controller or a base station. In one embodiment of theinvention, the first or the second packet switching node is a node,which performs the forwarding and switching of data packets at linklayer. The invention is not restricted to packet switching nodes thatswitch packets at network layer level in the manner of e.g. IP routers.By packets are meant herein throughout this disclosure data packetspertaining to any protocol layer, for example, network layer packets,link layer frames, Asynchronous Transfer Mode (ATM) cells.

In one embodiment of the invention, the logical link parameter exchangeis performed in response to the detection of handover condition at thefirst packet switching node.

In one embodiment of the invention, the first logical link layer entityin the mobile node is removed after the detecting of handovercompletion.

In one embodiment of the invention, the at least one ciphering parameteris received from the first packet switching node to the second packetswitching node when the first packet switching node requests handoverpreparation from the second packet switching node. This means that theat least one ciphering parameter is sent from the first packet switchingto the second packet switching in the message that requests handoverpreparation.

In one embodiment of the invention, the logical link layer informationis received from the first packet switching node to the second packetswitching node when the first packet switching node requests handoverpreparation from the second packet switching node. This means that thelogical link layer information is sent from the first packet switchingnode to the second packet switching in the message that requestshandover preparation.

In one embodiment of the invention, the logical link parameter exchangeis performed in response to the condition where the mobile node receivesan LLC frame, which has a duplicate flag set. The duplicate flagindicates the duplication of the LLC frame for handover purposes. In oneembodiment of the invention, the duplicate flag is only accepted by themobile node while handover is being performed. Otherwise, the receivingof the flag results in an error indication to the peer LLC-entity.

In one embodiment of the invention, the logical link layer means in themobile node and in the first and second packet switching nodes arerepresented by one or many Logical Link Control (LLC) entities, aLogical Link Management Entity (LLME) and a multiplexing entityassociated with them. On transmission the multiplexing entity generatesand inserts the FCS, performs a frame ciphering function and providesSAPI-based logical link control layer contention resolution between thevarious logical link entities. The functions performed by multiplexingentity and LLME are described in 3GPP specification 23.060.

In one embodiment of the invention, the control means in the first andsecond packet switching node comprise the higher protocol layer entitiesabove the logical link layer. For example, in a SGSN the control meansmay comprise the relay layer entities, the SNDCP layer entities and theGTP layer entities.

In one embodiment of the invention, the control means in the mobile nodecomprise the higher protocol layer entities pertaining to the GPRS userplane.

In one embodiment of the invention, the signaling means in the mobilenode comprise the signaling protocols used to communicate with the firstand the second packet switching nodes. In a GPRS mobile terminal thesignaling means comprises the GPRS control plane signaling protocolstack entities. In one embodiment of the invention, the actual mobilitymanagement and radio control related application logic are performed incontrol means or in separate control means in association with signalingmeans. In this embodiment the exchange of signaling messages is handledby separate means reserved for the task.

In one embodiment of the invention, the signaling means in the first andthe second packet switching nodes comprise the signaling protocols usedto communicate with the mobile node. In a SGSN the signaling meanscomprises the GPRS control plane signaling protocol stack entities.

In one embodiment of the invention, the sending of logical link layerframes or any other messages between the mobile node and the packetswitching nodes is performed via a radio access network so that theframes and messages are forwarded by one or many intermediate networkelements such as base station controllers, radio network controllers andbase transceiver stations. In one embodiment of the invention, the firstand the second packet switching nodes are directly connected to basetransceiver stations and manage the radio network control proceduresdirectly.

The benefits of the invention are associated with improved quality ofservice. With the invention it is now possible to provide a continuouspacket stream to a mobile station during handover.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and constitute a part of thisspecification, illustrate embodiments of the invention and together withthe description help to explain the principles of the invention. In thedrawings:

FIG. 1 is a block diagram illustrating the prior art architecture andthe protocol stacks in a General Packet Radio Service (GPRS) system inassociation with the GSM/EDGE Radio Access Network (GERAN);

FIG. 2 is a block diagram illustrating General Packet Radio Service(GPRS) network architecture and problems in prior art associated withduplicated packet forwarding;

FIG. 3 is a signaling diagram illustrating signaling during a packetswitched handover in prior art;

FIG. 4 is a signaling diagram illustrating the delay associated with asolution, which merely forwards packets from a source node to a targetnode during handover processing;

FIG. 5 is a signaling diagram depicting one embodiment of packetswitched handover method utilizing state transfer, according to theinvention;

FIG. 6 a is a block diagram depicting one embodiment of packet switchedhandover method utilizing frame forwarding via Serving GPRS Support Node(SGSN), according to the invention;

FIG. 6 b is a block diagram depicting one embodiment of packet switchedhandover method utilizing frame forwarding directly to target BaseStation Subsystem, according to the invention;

FIG. 7 is a signaling diagram depicting one embodiment of packetswitched handover method utilizing logical link parameter reset,according to the invention;

FIG. 8 is a block diagram depicting one embodiment of packet switchedhandover method utilizing duplicate logical link control entities,according to the invention;

FIG. 9 is a signaling diagram depicting one embodiment of packetswitched handover method utilizing a duplicate frame indicator,according to the invention;

FIG. 10 is a flow chart depicting one embodiment of packet switchedhandover method utilizing context transfer, according to the invention;

FIG. 11 is a flow chart depicting one embodiment of packet switchedhandover method utilizing frame forwarding, according to the invention;

FIG. 12 is a flow chart depicting one embodiment of packet switchedhandover method utilizing logical link reset, according to theinvention;

FIG. 13 is a flow chart depicting one embodiment of packet switchedhandover method utilizing duplicate logical link control entities,according to the invention;

FIG. 14 is a flow chart depicting one embodiment of packet switchedhandover method utilizing a duplicate frame indicator, according to theinvention;

FIG. 15 illustrates a Serving GPRS Support Node (SGSN) in one embodimentof the invention; and

FIG. 16 illustrates a mobile node in one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 10 is a flow chart depicting one embodiment of packet switchedhandover, which utilizes state transfer using a signaling illustrated inFIG. 5. The signaling is performed in GPRS system architecture, which isillustrated in FIG. 2. At step 1000 it is checked if handover occurs. Incase there is handover MS 100 sends radio quality measurementinformation pertaining to neighboring cells to source BSS 262 usingmessage 301. Based on the measurement information source BSS 262determines that handover is required. The determination is performedusing an algorithm that is executed in a Base Station Controller (BSC)within the source BSS 262. At time t₀ source BSS 262 determines thathandover is to be performed to a new cell, which is in the area of a newSGSN, which is SGSN 204. Source BSS 262 sends a PS Handover Requiredmessage 302 to old SGSN 202. The message comprises, for instance, thesource cell, the target cell, TLLI, cause and a transparent container.SGSN 202 determines based on the target cell if the handover is anintra- or inter-SGSN handover. SGSN 202 determines the identity of thenew SGSN and sends a Prepare PS Handover Request message 303 to SGSN204.

At step 1002 the state pertaining to the logical link is obtained by theLLC-entity in SGSN 204. This is achieved so that Prepare PS HandoverRequest message carries LLC state information element 500. LLC stateinformation element 500 comprises information that is used tosynchronize LLC-entities in SGSN 202 and SGSN 204. Information element500 comprises at least the session key K_(c), the IOV values for bothmodes of operation, both LFN values and the four OC values. SGSN 204stores information element 500 until SGSN 202 forwards packets to it.The handover signaling between network elements continues as explainedin association with FIG. 3.

At step 1004 when a first forwarded packet is received from SGSN 202, anLLC entity is initialized in SGSN 204. During initialization SGSN 204uses information element 500. By having information element 500 and theLLC state information in it, it is possible for SGSN 204 to construct anLLC-entity, which is an exact replica of the LLC-entity in SGSN 202 fromMS 100 point of view. Thereupon, MS 100 is able to receive LLC framesfrom both LLC-entities without noticing a difference. In one embodimentof the invention the LLC-entity in SGSN 204 is initialized and startedalready after SGSN 204 has received message 303 and no packets to beforwarded have yet been received by SGSN 204. At step 1006 SGSN 204starts forwarding packets received via SGSN 202 using the LLC-entityconstructed and initialized at step 1006.

FIG. 11 is a flow chart depicting one embodiment of packet switchedhandover, which utilizes frame forwarding in a system as illustrated inFIG. 6 a or 6 b. At step 1100 SGSN 202 waits for a message from sourceBSS 262 indicating that handover is required. In one embodiment of theinvention the handover indication may also be received from MS 100. Whenthe message is received method continues in step 1102. At step 1102 SGSN202 waits for an event where SGSN 202 receives a packet 610 from GGSN200, which is the first user plane packet after the start of handover.At this event a first LLC frame 614 that carries data from packet 610 isto be sent by SGSN 202. When the event occurs packet 610 is received byan SNDCP entity 600 in SGSN 202 via the GTP and relay layers asillustrated in FIG. 1.

Packet 610 is received to SGSN 202 via tunnel 240. The SNDCP entity 600performs packet segmentation for packet 610 and other SNDCP level tasksand issues a request to an LLC-entity 252 to send first LLC-frame 614.The request is issued in the form of an LLC Service Data Unit (SDU). Atstep 1104 LLC-entity 252 prepares an LLC-PDU using the informationcontained in LLC-SDU and the LLC-entity 252 state variables. At step1106 LLC-entity 252 sends the prepared LLC-PDU in a first LLC-frame 614to source BSS 262 and BSC 212 therein.

At step 1108 LLC-entity 252 passes the LLC-PDU in a second LLC-frame 616to a frame forwarding entity 604 in association with SNDCP entity 600.It should be noted that second LLC-frame 616 is a duplicate of LLC-frame614. Frame forwarding entity 604 sends the second LLC-frame 616 to SGSN204 using a connection 241, which tunnels LLC-frames prepared byLLC-entity 252 to SGSN 204. Connection 241 is, for example, a GTP tunnelestablished between SGSN 202 and SGSN 204 for the transparent forwardingof LLC-frames. The second LLC-frame 616 is received by LLC-entity 606 inSGSN 204. LLC-entity 606 is configured to receive LLC-frames viaconnection 241 and forward them transparently towards target BSS 264.The transparent forwarding means in this case that the LLC-entity doesnot alter the LLC-frame fields indicating LLC-entity 252 state. In oneembodiment of the invention, relay LLC PDU formed from LLC-frame 616 isnot relayed through SNDCP protocol entity in SGSN 204. In anotherembodiment of the invention the LLC PDU from LLC-frame 616 is relayedthrough protocol entity chain GTP-SNDCP-LLC-BSSGP in order to be sent totarget BSS 264.

In one embodiment of the invention illustrated in FIG. 6 b SGSN 202passes second LLC-frame 616 directly to target BSS 264. This is achievedso that a connection 241 b is formed between SGSN 202 and target BSS264. This is achieved so that at step 1108 is omitted from the method.Instead, at step 1110 LLC-entity 252 passes the LLC-PDU in a secondLLC-frame 616 to a frame forwarding entity 604 b in association withSNDCP entity 600. Frame forwarding entity 604 b sends the secondLLC-frame 616 to target BSS 264 using connection 241 b. Target BSS 264is configured to receive LLC-frame 616 and other duplicate LLC-framesfor handover and to prepare them for transmission to MS 100.

FIG. 12 is a flow chart depicting one embodiment of packet switchedhandover, which utilizes logical link reset achieved using a signalingillustrated in FIG. 7. The signaling is performed in GPRS systemarchitecture, which is illustrated in FIG. 2. At step 1200 it is checkedif handover occurs. In case there is handover MS 100 sends radio qualitymeasurement information pertaining to neighboring cells to source BSS262 using message 301. Based on the measurement information source BSS262 determines that handover is required. The determination is performedusing an algorithm that is executed in a Base Station Controller (BSC)within the source BSS 262. At time t₀ source BSS 262 determines thathandover is to be performed to a new cell, which is in the area of a newSGSN, which is SGSN 204. Source BSS 262 sends a PS Handover Requiredmessage 302 to SGSN 202. The message comprises, for instance, the sourcecell, the target cell, TLLI, cause and a transparent container. SGSN 202determines based on the target cell if the handover is an intra- orinter-SGSN handover. SGSN 202 determines the identity of a new SGSN,which in this case is SGSN 204, and sends a Prepare PS Handover Requestmessage 303 to SGSN 204.

At step 1202 cipher parameters pertaining to the logical link areobtained by the LLC-entity in SGSN 204. This is achieved so that PreparePS Handover Request message carries cipher parameter information element700. Information element 700 comprises, for example, the session keyK_(c) and any other parameters not re-negotiated at during XID-resetprocedure. At step 1204 SGSN 202 starts XID-reset procedure so thatLLC-entity 252 in SGSN 202 sends an XID command message 701 to MS 100via source BSS 262. XID command message 701 includes information on LLCparameters such as, for example, LLC version number, IOV values,retransmission timeout, maximum number of retransmissions, maximuminformation field lengths in the two acknowledgement modes, frame buffersizes in uplink and downlink direction, window sizes in uplink anddownlink directions and layer-3 parameters. XID command message 701proposes LLC parameter values that correspond to initial LLC values setwhen a new SGSN initializes its LLC-entity. At the receipt of XIDcommand message 701, MS 100 sets LLC parameters to the values proposedand issues XID response message 702 acknowledging the proposed parametervalues. In one embodiment of the invention MS 100 is configured toaccept the parameters proposed by SGSN 202 automatically when it isaware that a handover process is pending. In one embodiment of theinvention MS 100 accepts a downlink PDU automatically from SGSN 204 ifit is flagged accordingly and if it is received during handover.

At step 1206 SGSN 204 starts receiving packets forwarded from SGSN 202.In FIG. 7 such packets are carried in packet stream 308. SGSN 204initializes its LLC-entity 254 to have initial LLC parameter values. Theinitial values correspond to the LLC-parameter values negotiated betweenSGSN 202 and MS 100 during XID-reset procedure at step 1204. Thereupon,SGSN 204 starts sending the forwarded packets towards MS 100.Afterwards, SGSN 204 and MS 100 may negotiate more optimal LLCparameters. Typically the re-negotiation of LLC parameters is performedafter routing area update.

FIG. 13 is a flow chart depicting one embodiment of packet switchedhandover, which utilizes illustrated in FIG. 8. At step 1300 MS 800 hasonly one LLC-entity, which is first LLC-entity 802. First LLC-entity 802is the peer entity for LLC-entity 252 in SGSN 202. There is an LLCconnection 842 between LLC-entities 252 and 802. LLC connection 842carries a packet stream originating from GGSN 200 to MS 800. MS 800waits for a condition where handover is required. This is determinedbased on, for example, a handover command received from BSS 262. Whenthe condition is detected the method continues in step 1302. At step1302 MS 100 constructs a second LLC-entity 804, which existssimultaneously with first LLC-entity 802 at least during handover.Second LLC-entity 804 is the peer entity for LLC-entity 254 in SGSN 204.At step 1304 MS 800 initializes second LLC-entity 804. The LLCparameters are initialized to values compatible with the values to whichSGSN 204 initializes the LLC parameters while it initializes LLC-entity254 at step 1306. At step 1306 SGSN 204 receives packets forwarded fromSGSN 202 via a tunneling connection 241. Tunneling connection 241 is,for example, a GTP tunnel. SGSN 204 sends the forwarded packets towardsMS 800 using LLC connection 844, which it sets up between LLC-entities254 and 804. At step 1308 MS 800 checks if handover is finished. Ifhandover is not finished method continues at step 1308.

When the handover is finished LLC connection 842 between LLC-entities252 and 802 is no longer used to carry LLC-frames. In one embodiment ofthe invention at step 1310 MS 800 checks if LLC parameters pertaining toLLC connection 844 are suitable taking into consideration, for example,the radio conditions at the cell served by BTS 224. MS 800 may alsoreadjust the parameters depending on available memory and the data rateon LLC connection 844. In one embodiment of the invention LLC parametersat LLC-entity 254 are initialized first to moderate values, which aremade suitable for most mobile stations under different radio conditions.Mobile stations may have also varying memory sizes and softwareversions. For example, information field lengths, frame buffer andwindow sizes may be first set to values lower than would otherwise benegotiated between peering LLC-entities. If MS 800 determines that LLCparameters are not suitable, it readjusts them to different values atstep 1312. The parameters are to be readjusted, for example, using anXID reset procedure involving the exchanging of XID command and XIDresponse between LLC-entities 804 and 254. If parameter values aresuitable no readjusting is needed.

In one embodiment of the invention, MS 800 removes the first LLC-entity,which was used prior to handover, after the handover is complete. Atstep 1314 MS 800 performs the procedures necessary for removingLLC-entity 802, which is no longer used. MS 800 may also removeLLC-entity 802 directly after step 1308 before checking whether the LLCparameters are suitable. The removing of LLC-entity comprises, forexample, the releasing of memory reserved for the use of LLC-entity 802and LLC connection 842 in MS 800. Similarly, information pertaining toLLC-entity 802 and LLC connection 842 may be removed from memory tablesmaintained in MS 800.

FIG. 14 is a flow chart depicting one embodiment of packet switchedhandover method, which utilizes a duplicate frame indicator conveyed andprocessed using a signaling illustrated in FIG. 9. The signaling isperformed in GPRS system architecture, which is illustrated in FIG. 2.At step 1400 it is checked if handover occurs. In case handover occursMS 100 sends radio quality measurement information pertaining toneighboring cells to source BSS 262 using message 301. Based on themeasurement information source BSS 262 determines that handover isrequired. The determination is performed using an algorithm that isexecuted in a Base Station Controller (BSC) within the source BSS 262.At time t₀ source BSS 262 determines that handover is to be performed toa new cell, which is in the area of a new SGSN, which is SGSN 204.Source BSS 262 sends a PS Handover Required message 302 to SGSN 202. Themessage comprises, for instance, the source cell, the target cell, TLLI,cause and a transparent container. SGSN 202 determines based on thetarget cell if the handover is an intra- or inter-SGSN handover. SGSN202 determines the identity of a new SGSN, which in this case is SGSN204, and sends a Prepare PS Handover Request message 303 to SGSN 204.

At step 1402 cipher parameters pertaining to the logical link areobtained by the LLC-entity in SGSN 204. This is achieved so that PreparePS Handover Request message carries cipher parameter information element900. Information element 700 comprises, for example, the session keyK_(c) and any other parameters not re-negotiated at during a XID-resetprocedure.

At step 1404 SGSN 204 waits for packets forwarded from SGSN 202 to it.When such a packet is received in message 308, the method continues atstep 1406. At step 1406 an SNDCP entity in SGSN 204 indicates toLLC-entity in SGSN 204 while requesting the sending of an LLC-SDU thatthe LLC-SDU is a first LLC-SDU comprising data from packets forwardedfrom SGSN 202 to SGSN 204. The LLC-PDU is therefore a duplicate ofanother LLC-PDU sent from SGSN 202. LLC-entity in SGSN 204 sets aduplicate for handover flag in the header of the LLC-PDU to be sent. Theflag may be carried in, for example, in one of the reserved bits in LLCaddress field or in one of the UI control field bits. Therefore, noextra bits are needed in LLC-PDU header. LLC parameters are set todefault handover values. The default values may be standardized so thatoptimization is maximized or normal default values are used. When MS 100receives the LLC-PDU in an LLC frame, it detects that the duplicate forhandover bit is set. At step 1408 MS performs implicit XID-reset for theLLC-entity in it. In implicit XID-reset the MS 100 sets automaticallythe LLC parameters to values, which are compatible with values set byLLC-entity in SGSN 204 when it is first created and initialized.Implicit XID-reset is required in MS 100 before it is able to processany LLC frames from SGSN 204. For example, this is due to the differingciphering parameters, for example overflow count, which have not beenreceived at step 1402.

FIG. 15 illustrates a Serving GPRS Support Node (SGSN) in one embodimentof the invention. SGSN 1500 comprises a signaling entity 1504, whichcommunicates with a logical link layer entity 1506. Signaling entity1504 performs GPRS control plane signaling. Logical link layer entity1506 carries both control plane and user plane messages as specified in3GPP 23.060 pertaining to LLC. In the embodiment of the inventiondisclosed in association with the description of FIGS. 6 and 11 logicallink layer entity 1506 is responsible for forming logical link layerProtocol Data Units (PDU) and sending the logical link layer ProtocolData Units (PDU) to new SGSN. In one embodiment of the invention thesending of the logical link layer PDUs to new SGSN is achieved so thatlogical link layer entity 1506 passes the PDUs to control entity 1502,which sends them via, for example, a GTP entity 1510 to the new SGSN. Inone embodiment of the invention signaling entity 1504 is responsible fordetecting handover conditions, requesting handover preparation fromother SGSNs, receiving handover preparation requests from other SGSNs,sending logical link layer state information, ciphering parameters andother information to other SGSNs. In one embodiment of the invention,the actual mobility management and radio related application proceduresassociated with signaling messages received to signaling entity 1504 areperformed by control entity 1502 or by a separate control entity withinsignaling entity 1504. In one embodiment of the invention control entity1502 is responsible, for example, for setting the state in logical linklayer entity 1506 based on logical link layer information received fromanother SGSN and sending logical link layer frames to mobile node duringhandover. The actual sending of logical link layer frames is performedvia lower protocol layers 1508. The arrows in FIG. 15 illustratedirections of information flows between the entities within SGSN 1500.

FIG. 16 illustrates a mobile node in one embodiment of the invention. InFIG. 16 mobile node is more specifically a GPRS mobile terminal. Mobilenode 1600 comprises a signaling entity 1604, which communicates with alogical link layer entity 1606. Logical link layer entity 1606 carriesboth control plane and user plane messages as specified in 3GPP 23.060.In one embodiment of the invention signaling entity 1604 is responsiblefor receiving signaling messages from the base station subsystem anddetects handover conditions and handover completion based on receivedsignaling messages. Logical link layer entity 1606 performs the LogicalLink Control (LLC) protocol related tasks. In the embodiment of theinvention disclosed in association with the description of FIG. 12logical link layer entity 1606 is arranged to renegotiate logical linklayer parameters with new SGSN after the handover completion. Mobilestation 1600 comprises also a control entity 1602, which performs higherprotocol layer related tasks and overall coordination of communication.In one embodiment of the invention control entity 1602 is arranged toform a first logical link layer entity during connection establishmentprocedure and a second logical link layer entity in response to ahandover condition. The arrows in FIG. 15 illustrate directions ofinformation flows between the entities within mobile node 1600.

It is obvious to a person skilled in the art that with the advancementof technology, the basic idea of the invention may be implemented invarious ways. The invention and its embodiments are thus not limited tothe examples described above; instead they may vary within the scope ofthe claims.

1. A method of performing packet switched handover in a mobile communication network, the method comprising: detecting a handover condition associated with a mobile node in a first packet switching node; requesting handover preparation by said first packet switching node from a second packet switching node; receiving logical link layer information from said first packet switching node to said second packet switching node; setting a state in a logical link layer entity in said second packet switching node based on said logical link layer state information; and sending logical link layer frames from said first and second packet switching nodes to said mobile node during the handover.
 2. The method according to claim 1, further comprising: utilizing a general packet radio service (GPRS) network as said mobile communication network, serving GPRS support nodes (SGSN) as said first and second packet switching nodes, and GPRS logical link control (LLC) as said logical link layer.
 3. A method of performing packet switched handover in a mobile communication network, the method comprising: detecting a handover condition associated with said mobile node in a first packet switching node; requesting handover preparation by said first packet switching node from a second packet switching node; receiving a packet at said first packet switching node; forming a logical link layer protocol data unit (PDU) from data in said packet; sending a first frame containing said logical link layer protocol data unit (PDU) to said mobile node from said first packet switching node; sending said logical link protocol data unit (PDU) from said first packet switching node to said second packet switching node; and sending a second frame containing said logical link layer protocol data unit (PDU) to said mobile node from said second packet switching node.
 4. The method according to claim 3, further comprising the step of: utilizing a general packet radio service (GPRS) network as said mobile communication network, Serving GPRS support nodes (SGSN) as said first and second packet switching nodes, and GPRS logical link control (LLC) as said logical link layer.
 5. The method according to claim 3, further comprising: utilizing a general packet radio service (GPRS) network as said mobile communication network, a serving GPRS support node (SGSN) as said first packet switching node, a base station subsystem (BSS) node as said second packet switching node, and GPRS logical link control (LLC) as said logical link layer.
 6. A method of performing packet switched handover in a mobile communication network, the method comprising: detecting a handover condition associated with a mobile node in a first packet switching node; requesting handover preparation by said first packet switching node from a second packet switching node; receiving at least one ciphering parameter from said first packet switching node to said second packet switching node; performing a logical link parameter exchange between said mobile node and said first packet switching node; and sending logical link layer frames from said first and second packet switching nodes to said mobile node during handover.
 7. The method according to claim 6, wherein, in said performing step, said logical link parameter exchange is performed in response to a condition where said mobile node receives a logical link layer frame, which has a duplicate flag set.
 8. The method according to claim 6, further comprising: utilizing a general packet radio service (GPRS) network as said mobile communication network, serving GPRS support nodes (SGSN) as said first and second packet switching nodes, GPRS logical link control (LLC) as said logical link layer, and logical link control (LLC) exchange identification (XID) negotiation as said logical link parameter exchange.
 9. A method of performing packet switched handover in a mobile communication network, the method comprising: forming a first logical link layer entity in a mobile node; detecting a handover condition in said mobile node; forming a second logical link layer entity in said mobile node; sending logical link layer frames from a first and a second packet switching nodes to said mobile node during handover; detecting handover completion; and renegotiating logical link layer parameters between said mobile node and said second packet switching node after said detecting of said handover completion when the logical link layer parameters are not suitable.
 10. The method according to claim 9, further comprising: removing said first logical link layer entity in said mobile node after said detecting of handover completion.
 11. The method according to claim 9, further comprising: utilizing a general packet radio service (GPRS) network as said mobile communication network, serving GPRS support nodes (SGSN) as said first and second packet switching nodes, and GPRS logical link control (LLC) as said logical link layer.
 12. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: first signaling means in said first packet switching node for detecting a handover condition associated with said mobile node, requesting handover preparation from said second packet switching node and sending logical link layer information to said second packet switching node; second signaling means in said second packet switching node for receiving logical link layer information from said first packet switching node; first control means in said second packet switching node arranged for setting the state in a logical link layer entity based on logical link layer information from said first packet switching node; and second control means in said first packet switching node arranged for sending logical link layer frames to said mobile node during handover.
 13. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: signaling means in said first packet switching node for detecting a handover condition associated with said mobile node and requesting handover preparation from said second packet switching node; first logical link layer means in said first packet switching node for forming logical link layer protocol data units (PDU) and sending said logical link layer protocol data units (PDU) to said second packet switching node; and second logical link layer means in said second packet switching node for sending said logical link layer protocol data units (PDU) transparently to said mobile node.
 14. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: first signaling means in said first packet switching node for detecting a handover condition associated with said mobile node, requesting handover preparation from said second packet switching node and sending at least one ciphering parameter to said second packet switching node; second signaling means in said second packet switching node for receiving at least one ciphering parameter from said first packet switching node; and logical link layer means in said first packet switching node for performing a logical link parameter exchange with said mobile node.
 15. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: control means in said mobile node arranged for forming a first logical link layer entity in response to connection establishment and a second logical link layer entity in response to a handover condition; signaling means in said mobile node for detecting the handover condition and a handover completion; and logical link layer means in said mobile node arranged for renegotiating logical link layer parameters with said second packet switched node after the handover completion when the logical link layer parameters are not suitable.
 16. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: a transceiver in said first packet switching node configured to detect a handover condition associated with said mobile node, requesting handover preparation from said second packet switching node and send logical link layer information to said second packet switching node; a receiver in said second packet switching node configured to receive logical link layer information from said first packet switching node; a first controller in said second packet switching node configured to set the state in a logical link layer entity based on logical link layer information from said first packet switching node; and a second controller in said first packet switching node configured to send logical link layer frames to said mobile node during handover.
 17. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: a transceiver in said first packet switching node configured to detect a handover condition associated with said mobile node and request handover preparation from said second packet switching node; a first logical link layer in said first packet switching node configured to form logical link layer protocol data units (PDU) and sending said logical link layer protocol data units (PDU) to said second packet switching node; and second logical link layer in said second packet switching node for sending said logical link layer protocol data units (PDU) transparently to said mobile node.
 18. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: a transceiver in said first packet switching node configured to detect a handover condition associated with said mobile node, request handover preparation from said second packet switching node and send at least one ciphering parameter to said second packet switching node; a receiver in said second packet switching node configured to receive at least one ciphering parameter from said first packet switching node; and a logical link layer in said first packet switching node for performing a logical link parameter exchange with said mobile node.
 19. A system comprising a mobile node, a first and a second packet switching node, the system further comprising: a controller in said mobile node configured to form a first logical link layer entity in response to connection establishment and a second logical link layer entity in response to a handover condition; a detector in said mobile node configured to detect the handover condition and a handover completion; and a logical link layer in said mobile node configured to renegotiate logical link layer parameters with said second packet switched node after the handover completion when the logical link layer parameters are not suitable. 